Macrolide antibiotics play a therapeutically important role, particularly with the emergence of new pathogens. Structural differences are related to the size of the lactone ring and to the number and nature (neutral or basic) of the sugars. Macrolides are classified according to the size of the lactone ring (12, 14, 15 or 16 atoms). The macrolide antibiotic family (14-, 15- and 16-membered ring derivatives) shows a wide range of characteristics (antibacterial spectrum, side-effects and bioavailability). Among the commonly used macrolides are erythromycin, clarithromycin, and azithromycin. Macrolides possessing a 3-oxo moiety in place of the 3-cladinose sugar are known as ketolides and have shown enhanced activity towards gram-negative bacteria and macrolide resistant gram-positive bacteria. The search for macrolide compounds which are active against MLSB-resistant strains (MLSB=Macrolides-Lincosamides-type B Streptogramines) has become a major goal, together with retaining the overall profile of the macrolides in terms of stability, tolerance and pharmacokinetics.